The simplest protection method for battery packs is a fuse that opens if the system draws excessively high current. However, conventional one-time fuses open permanently and render the battery useless once the filament is “blown.” A more useful protection device is a positive temperature-coefficient resistor switch that becomes a high resistance for excessive currents, inhibiting the current flow. Then, when it cools, the resistance reverts to its low resistance, allowing normal operation to resume.
A more complex protection circuit is found in some battery packs. These are IC-based solutions integrated within the battery pack. The battery-protection ICs prevent excessive current, which could lead to high heat. Cells in a lithium-ion (Li-ion) pack may need independent voltage monitoring. The more cells connected in series, the more complex the protection circuit.
The protection circuit limits the maximum charge and discharge current and monitors the cell temperature. This protects against overvoltage, undervoltage, overcharge current, and overdischarge current in battery packs. Be careful not to discharge a lithium-based battery below 2.5 V, because it may cut off the battery’s protection circuit.
Most manufacturers don’t sell the Li-ion cells individually, but package them in a battery pack, complete with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond its safe limits.
Ideally, the protection circuit should consume no current when the battery-powered system is turned off. However, the protector always consumes some small current.
Seiko’s S-8211C series protection ICs target single-cell Li-ion/Li-polymer rechargeable batteries. These devices include high-accuracy voltage detectors and delay circuits (see the figure). They’re suitable for protecting single-cell rechargeable Li-ion/Li-polymer battery packs from overcharge, overdischarge, and overcurrent.
In the operation mode, current drain is 3.0 µA (typical) and 5.5 µA (maximum) at 25°C. Power-down current is 0.2 µA (maximum) at 25°C. This lead-free product comes in SOT23-5 and SNT-6A packages.